Abstract

Magnesium is one of the most important minerals in the body and it participates in many essential processes, such as energy metabolism and protein synthezis. Magnesium deficiency in cattle may occur when they are let out for pasture in the spring, and is associated with tetany, known as grass staggers. A high yielding dairy cow often loses more magnesium daily in the milk than she has available in the body, and magnesium thereby needs to be provided in the feed to prevent the cow from being affected by deficiency symptoms.

The uptake of magnesium is mainly located to the rumen and is not regulated by hormones. The surplus is excreted in the urine by the kidneys. The digestibility of magnesium in common feedstuff is about 20%. Many studies, mainly on dry cows or sheep, have shown that the absorption of magnesium in the rumen is negatively affected by the presence of potassium ions. To compensate the lower uptake, extra magnesium can be supplied when the potassium content in the diet is high.

The purpose of this study was to investigate how the magnesium balance in lactating cows was affected by increasing levels of potassium in the diet, and to determine in which level of potassium extra magnesium needed to be provided. The hypothesis was that the cows would absorb less magnesium when fed diets with increasing potassium concentration, and that this would be compensated for by increasing levels of magnesium.

Six lactating cows were fed two levels of magnesium (1,9 and 3,7 g/kg DM) and three levels of potassium (19, 28 and 37 g/kg DM) during six experimental periods in a Latin square design. Spot samples of feces, urine and milk were analyzed for magnesium to be able to calculate the magnesium balance. Samples of the blood and the rumen fluid were also taken. The pH was measured in the urine and the rumen fluid, and the consistency of the feces was estimated. Creatinine in the urine and acid insoluble ash in the faeces were used as markers to calculate the daily amount of urine and faeces produced.

The results showed that the cows excreted more magnesium in the urine when they were given extra magnesium in their diets. This indicates that they absorbed more magnesium from the rumen. The concentration of magnesium in the rumen fluid and in the plasma was higher when the cows were fed increased levels of magnesium. The concentration of magnesium in milk was not affected by treatment. The absorption, calculated as the proportion of the intake that was excreted in urine and milk, was lower when the diets were supplied with extra magnesium, 12,1 % vs 16,4 %. The feces had a more soft consistency when the magnesium intake was higher. The level of potassium had no influence on the magnesium balance, but had an impact on the rumen and urine pH. The calculated absorption of magnesium was lower than what has been showed by earlier studies. The lack of effect of increased potassium concentration can be due to the chosen levels of magnesium and potassium in the experimental diets. Another reason could be the differences between lactating cows and dry cows or sheep.